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Downy mildew

Causal organism: Fungus
Important species

  • Downy mildew on tobacco, crucifers, spinach, soybeans, alfalfa, onion, and many herbaceous and woody plants (Peronospora spp.)
  • Downy mildew on cucurbits (Psuedoperonospora spp.)
  • Downy mildew on sorghum and corn (Peronosclerospora spp.)
  • Downy mildew on grasses and millet (Sclerospora spp.)
  • Downy mildew on corn, rice, wheat (Sclerophthora spp.)
  • Host plants

    Beans, crucifers, cucurbits, maize, potato, tomato, rice, and a wide range of herbaceous and woody plants

    Affected plant stages

    Seedling, vegetative, reproductive, and post harvest

    Affected plant parts

    Leaves, inflorescence, fruits, and seeds


    Downy mildew infection begins as angular yellow spots on the upper leaf surface. Then they become brilliant-yellow. Eventually, the internal parts of these spots become brown with yellow margins. The underside of this infected leaf has fine, grayish fungal growth. Infected young shoots, fruits, and seeds have white coating of fungal spores.

    On cereals, the most striking symptom is the long and narrow chlorotic streaks (dead spots) on the leaves. During the early morning, downy fungal growths are often seen on the undersides of the leaves.

    On crucifers, the disease initially appears as irregular yellow patches on the leaves. These lesions later turn tan to light-brown. During cool and humid weather condition, the fungus develops white fungal growths on the undersides of leaves. Heavily infested leaves will have a blighted appearance as a result of numerous infection sites. Early symptoms on transplants may resemble bacterial leaf spot. Severely diseased seedlings are stunted that will die eventually.

    On cucurbits except watermelon, small yellowish spots occur on the upper surface of the leaves. In the later stage of infection, the colorations become brilliant-yellow with the internal parts of the spots turning brown. Usually, the spots are angular as they are somewhat restricted by the small leaf veins. When the leaves are wet, a downy white or gray to light-blue fungus growth can be seen on the underside of individual spots. Downy mildew infects only the leaves on cucurbits.

    On onions and garlic, infected plants have fine, grayish-white or purple fruiting bodies on the surface of older leaves. The underneneath of the infected leaf tissues becomes pale-green, then turns yellow that will finally collapse. The diseased-plants have large, yellowish, and circular fungal masses. The yellowing patterns often enlarge in the direction of the prevailing winds.

    On soybean, the first symptom is the appearance of indefinite, yellowish-green spots on the upper leaf surface. These spots later enlarge and turn pale to bright-yellow or grayish-brown or dark-brown that are surrounded by yellowish-green margins. Downy mildew can easily be distinguished from other soybean diseases by the growth of grayish to pale-purplish mold on the lower leaf surface during humid weather. Severely infected leaves turn yellow, then brown, curl at the edges, and drop early. The causal fungus also grows within the pods and may produce whitish, crusty growth of spores on the seeds. The disease causes defoliation and lowers the seeds' size.

    Conditions that favor development

    1. Cool moist and warm moist weather conditions
    2. Weeds found in between the crops
    3. Infected leaves that are left to rot in the field
    4. Poor plant aeration

    Prevention and control

    1. Select and use only diseased-free seeds for sowing. If possible, procure seeds that are resistant to downy mildew.
    2. Transplant only healthy seedlings.
    3. Proper land preparation to make sure that your soil is well drained.
    4. Provide adequate plant spacing, between the rows and the hills, to reduce the density of the canopy and minimize humidity. Pruning of new growth also helps proper plant's aeration.
    5. Remove infested plants and prune infested shoots. However, do these sanitation practices when the plants are not wet to prevent further spread of the disease. Properly dispose collected diseased-parts either by burning or burying them.
    6. Avoid overhead watering. It lengthens the duration of leaf wetness and favors further development of the disease.
    7. After harvest, plow-under all the plant debris.
    8. Practice crop rotation. Make a list of the crops that are susceptible to downy mildew. Rotate these with resistant ones.
    9. Bordeaux mix
    10. Garlic extract
    11. Seed treatment

    External links


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